1. Field of the Invention
This invention generally relates to systems and methods for heating and/or treating a stream containing oxidizable matter, particularly reactor systems and methods utilizing oxidation reactions at or near supercritical conditions for water. Such systems and methods may be particularly useful to treat organic waste in aqueous-organic waste streams.
2. Description of the Related Art
The treatment of organic waste is becoming increasingly important. Traditional means of handling organic waste, such as landfill isolation or incineration, have many drawbacks. These drawbacks include landfill leakage and air pollution from incomplete combustion during incineration.
Concerns over problems associated with traditional organic waste treatment solutions have led to new developments in waste treatment. One promising solution for the treatment of organic matter is the accelerated oxidation of organic matter in an aqueous stream at elevated temperatures and pressures. This treatment has been discovered to be especially efficient when reacting the organic matter with an oxidant at or near supercritical conditions for water. The thermodynamic critical point for water is a temperature of about 705.degree. F. at a pressure of about 3200 psia. Above these conditions, water is supercritical.
The process of reacting organic matter with an oxidant at supercritical conditions for water is referred to as supercritical water oxidation. At temperatures and pressures above its thermodynamic critical point, water achieves a state similar to a dense gas. Some of the properties of supercritical water that are advantageous for the oxidation of organic matter are relatively low viscosity and relatively high organic solubility in a substantially single, dense phase that is similar to high-pressure, superheated steam. Thus, supercritical water tends to provide a dispersed reaction medium in which organic matter will oxidize with high efficiency when in the presence of an oxidant.
Supercritical water oxidation of organic matter generally produces carbon dioxide and water. These reaction products are generally easily separated and are non-toxic to the environment. Supercritical water oxidation achieves high destruction efficiencies of organic materials. Efficiencies of 99.99% or greater have been obtained. These high efficiencies and clean reaction products make supercritical water oxidation a significant technology for waste treatment.
Specially designed reactor systems have been developed for treatment of aqueous-organic streams by supercritical water oxidation. In a typical supercritical water oxidation system the aqueous-organic stream is fed into a reactor at conditions around the critical temperature for water. An oxidant (typically oxygen) is introduced in the reactor. The organic matter reacts with the oxidant and produces an effluent. The oxidation reaction produces a substantial amount of heat which results in the effluent being at a temperature significantly higher than the temperature of the aqueous-organic input stream.
An aqueous-organic input stream may typically be drawn from a holding tank and pressurized to above the critical point for water. A pump, such as a positive displacement pump, is used to pressurize the stream. A heat exchanger may be used to heat the input stream using heat from effluent from the reactor. Generally, an additional preheater is required to raise the temperature of the input stream to near the critical temperature for water when the stream enters the reactor. The preheater may be a gas-fired heater which continuously operates to heat the input stream to the reactor.
After the organic matter reacts with oxidant in the reactor, an effluent exits the reactor and flows through the heat exchanger. The system may include an effluent boiler to produce steam which may be used for electricity generation. The effluent may then flow through a cooler and control valve to further lower the effluent temperature and pressure. A liquid gas separator is typically used to separate the effluent into liquid and gas phases. The liquid effluent generally includes water saturated with carbon dioxide. The effluent gas may typically include about 90-95% carbon dioxide and 5-10% oxygen and be saturated with water.
The use of high pressures at elevated temperatures presents a serious problem in that it is difficult to construct a reactor or reaction chamber which can withstand supercritical water conditions. Generally as the temperature increases, the strength of construction material decreases. Supercritical pressures (generally greater than about 3,200 psia) at temperatures exceeding about 1,000.degree. F. present an enormous challenge for any construction material, let alone even higher pressures (of the order of 10,000 psia) and temperatures. Such high pressures and temperatures may be desirable for a number of reasons, including dissolution of inorganic salts under supercritical conditions. The harsh corrosive environment inside the reaction chamber also presents a serious design challenge.
Maintaining control of the reaction temperature is important. In some instances, the exothermic reactions proceed so rapidly that, unless controlled, they generate temperatures which endanger the integrity of the reaction vessel. It is important to control the temperature of the reaction to ensure that the organic matter is fully oxidized. The reaction temperature for proper oxidation is a function of the type and concentration of organic matter in the input stream. If the temperatures within the reactor are allowed to fall under certain limits, the reaction products may be incomplete, new phases may be formed in the reaction zone, or the reaction may cease to take place altogether.
Many of the following patents and patent applications relate to supercritical water oxidation methods and/or systems:
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